Printed circuits containing one or more circuit innerlayers are in prominent use today as demand increases for further and further weight and space conservation in electronic devices. Multilayer PCB's are constructed by layering imaged conductive layers of copper with dielectric layers to make a multilayer sandwich. The dielectric layers are usually organic resin layers that bond the copper layers together. Typically, the layers of copper and dielectric are bonded together by the application of heat and pressure. While the surface of the copper is smooth, it does not easily bond to the dielectric layer.
Successful fabrication of multi-layer printed circuit boards requires bonding together of copper and resin layers. However, direct bonding of copper and resin layers does not always provide sufficient bonding strength. It is therefore common to improve copper-resin bonding strength by providing surface roughness to the copper surface, thereby enhancing mechanical strength between the copper and resin layers. Surface roughness may be provided for example by mechanical cleaning (i.e., using buffing or scrubbing) or by chemical cleaning.
In one method of chemical treatment, surface roughness is provided to the copper surface by depositing an oxide layer on the copper surface, such as cuprous oxide, cupric oxide, or the like. Formation of the oxide layer, which turns the pink copper surface a brown-black color, creates minute unevenness on the copper surface, causing an interlocking effect between the copper surface and resin, thereby improving bonding strength.
Another method of improving the adhesion of dielectric material to a copper circuit trace uses a microetching technique. In microetching, no portion of the copper (e.g., copper circuitry traces) is completely etched away. Instead, the surface is etched (or oxidized) only to a limited extent so as to leave intact the original pattern of the copper being etched. Typically, the surface of the copper is etched only to a depth of between about 20 to about 500 μinches, as measured from the original surface to the depths of the microetching. This can be accomplished for example by choosing an appropriate microetching composition and limiting the extent of etching according to the parameters of the etching solution (including concentration, temperature, composition, etc.).
Low metal etch depths are advantageous for at least three reasons. First, a low etch depth removes less metal from the surface thereby leaving more of the original metal cross section intact. This is particularly important for circuit traces with impedance or resistance tolerances which must be maintained since these properties are directly related to the cross sectional area of the circuit. Second, low metal etch depths allow the opportunity for reworking defective parts. Lastly, low metal etch depths reduce the rate at which metal builds up in the adhesion promoting composition. Since metal build up in the microetching composition has an effect upon the ultimate useable life of the composition, lower etch depths lead to an extended useable life for the microetching solutions in terms of the maximum square feet of metal processable per gallon of the microetching composition.
Microetching and conversion coating solutions may be composed of hydrogen peroxide and an inorganic acid, such as sulfuric acid and phosphoric acid, as described for example in U.S. Pat. No. 5,800,859 to Price et al. and in U.S. Pat. Nos. 7,186,305, 6,554,948 and 6,383,272 all to Ferrier, the subject matter of each of which is herein incorporated by reference in its entirety.
Another type of microetching solution utilizes a cupric ion source, an organic acid, and a halide ion source, as described for example in U.S. Pat. No. 6,426,020 to Okada et al. and U.S. Pat. No. 5,807,493 to Maki et al., the subject matter of each of which is herein incorporated by reference in its entirety.
Another alternative oxide coating process is described in U.S. Pat. No. 7,351,353 to Bernard et al., the subject matter of which is incorporated herein by reference in its entirety. The Bernard patent describes a method and composition for providing roughened copper surfaces suitable for subsequent multilayer lamination. The method involves contacting a smooth copper surface with an adhesion promoting composition which includes an oxidizer, a pH adjuster, a topography modifier, and either a coating promoter or a uniformity enhancer.
While alternative conversion coating processes are advantageous over conventional oxide coating processes for a variety of reasons, the roughened copper surfaces formed by such processes exhibit chemical sensitivity and thus tend to be susceptible to chemical attack. Chemical attack typically occurs during post lamination processing steps. After a multilayer copper and dielectric sandwich is formed through the lamination process, certain post lamination processing steps are performed to prepare the multilayer PCB.
For example, “through-holes” are drilled through the multilayer sandwich in order to connect the inner layers of the circuit board. The act of drilling these holes typically leaves traces of resin smear on the through-hole interconnections that must be removed by a desmear process. One desmear process involves the application of a solvent sweller and a permanganate etch which can chemically attack the bond between the copper surface and dielectric resin at the site of the through holes. The permanganate etch is typically followed by an acid neutralizer which can chemically attack the bond and cause delamination. While other through-hole cleaning techniques are known, such as plasma etch or laser ablation, these processes generate intense heat which can also attack the copper/resin interface.
Once the desmear process is completed, the drilled holes are made conductive through direct metallization or similar processes. These processes involve numerous alkaline and acid processing steps, all of which can chemically attack the copper/resin interface. Further, the conductive through-hole is usually sealed with a layer of electrolytic copper. The electrolytic process involves alkaline and acidic baths which can also lead to chemical attack of the through-hole interconnects. The result of these chemical attacks may be the delamination of the sandwich layers in the area of the through holes.
The chemically attacked area is termed “pink ring” or “wedge void” in the circuit board industry. The formation of pink rings or wedge voids represents serious defects in the PCB's, especially in an era when increasingly high quality and reliability are demanded in the PCB industry.
Thus, there is a need for an improved oxide coating process that provides a surface that is less susceptible to chemical attack during post-lamination processing steps. In addition, there is a need for an improved oxide coating process that provides improved acid resistance during post-lamination processing steps.
Furthermore, there remains a need in the art for improved microetching compositions that can provide the desired degree of microetching in a copper or copper alloy surface while overcoming some of the difficulties noted in the prior art.
To that end, the inventors of the present invention have discovered that the use of a thiophosphate in the microetching composition, along with other additional optional additives, provides beneficial results with respect to resistance to chemical attack and improved acid resistance as compared with compositions of the prior art.